1. Field of the Invention
The present invention relates generally to an image reading apparatus for applying light to an original document and reading a document image. More particularly, the invention relates to an image reading apparatus which is capable of sharply reading a transparent document.
2. Related Background Art
FIG. 6A is a schematic diagram illustrating a conventional image reading apparatus. In the drawing, there is shown an image sensor 106 serving as photoelectric conversion means, such as a charge coupled device (CCD) for converting information concerning a scanned image into an electric signal, the sensor 106 being disposed within the main unit (apparatus unit) 100 of an image reading apparatus.
An original-document-mounting glass 101, used as a transparent original-document-mounting table, is disposed on the top surface of the apparatus unit 100. A document P placed on the surface of the document-mounting glass 101 is scanned by a scanning optical system 102 serving as scanning means so as to expose image information onto the image sensor 106.
Disposed within the image sensor 106 are three rows of sensors provided with filters of three colors, such as red (R), green (G) and blue (B), respectively, thereby performing color separation when reading a document image. The image scanning means 102 is constructed of a lamp unit 103 and a mirror unit 104, both of which are moved parallel to the glass 101 to perform scanning, and a stationary lens 105 fixed in the apparatus unit 100.
The lamp unit 103 is formed of a white-color light source L.sub.1 for illuminating the document P, and a first mirror M.sub.1 for reflecting the reflected light from the image formed on the document P toward the mirror unit 104. The mirror unit 104 is comprised of second and third mirrors M.sub.2 and M.sub.3, respectively, for returning the image light reflected by the first mirror M.sub.1 toward the image sensor 106.
The lamp unit 103 moves at a velocity twice as fast as the mirror unit 104 to perform scanning in order to ensure a constant optical path in the overall image reading region. Accordingly, the scanning optical system 102 of the above type is referred to as a "2:1 scanning optical system". These units 103 and 104 perform scanning (sub-scanning) using a driving source (not shown), such as a pulse motor, as a power source, in synchronization with the reading cycle of the image sensor 106.
In FIG. 6A, there is also shown a transparent-document-reading light source unit 200 serving as illumination means for reading transparent documents. Disposed within the light source unit 200 are a light source L.sub.2 located parallel to the light source L.sub.1 within the apparatus unit 100, and a light-diffusing translucent plate 201 placed to opposedly face the document-mounting table 101. The transparent-document-reading light source unit 200 is attached to the rear end of the image reading apparatus and pivots about a hinge 202.
For reading a transparent document, the light source L.sub.2 is driven by a driving source (not shown) to scan the area covered by the document-mounting glass 101 in a direction parallel to the translucent plate 201 while synchronizing with the image scanning means 102 of the apparatus unit 100. During this scanning operation, the light source L.sub.1 within the apparatus unit 100 is switched off. Light emitted from the light source L.sub.2 is diffused in the translucent plate 201, bringing about a light distribution illustrated in FIG. 6B (enlarged from the region D1 shown in FIG. 6A) on the document surface. From the light distributed as indicated in FIG. 6B, the light located on the light path in an area from the reading position of the image reading apparatus unit 100 to the image sensor 106 penetrates the document placed at a position P shown in FIG. 6A and is directed to the image sensor 106.
For reading a transparent document, the document is not allowed to be placed in the area A (hereinafter referred to as "the document-placing prohibited area A") at the upper edge of the document-mounting table 101. Prior to document reading, in this area A the image sensor 106 reads the light quantity and the light distribution directly obtained from the transparent-document light source L.sub.2 and uses these as data concerning, for example, shading correction.
FIG. 7 schematically illustrates a document image being formed on photodetectors of the image sensor 106. Photodetectors of three colors 106R, 106G and 106B are spaced apart from each other because a portion for accumulating charges photoelectrically converted by the photodetectors and a portion for transferring signals to an output stage are adjacently disposed around the photodetectors 106R, 106G and 106B of the image sensor 106.
Since the image sensor 106 is shifted relative to the document to allow the photodetectors 106R, 106G and 106B to read the same position of the document, the intervals between the photodetectors 106R, 106G and 106B are determined to be integral multiples of the width of the photodetectors 106R, 106G and 106B. If there is an m-line interval between the photodetectors 106R and 106G and an n-line interval between the photodetectors 106G and 106B, an image signal G representing one line of a document image is read m lines later relative to an image signal R, and an image signal B is read (m+n) lines later relative to the image signal R.
FIG. 8 is a block diagram illustrating the processing of image data read by the color image sensor 106. After the image data items of the respective colors read by the image sensor 106 are sent to and amplified in amplifiers 121R, 121G and 121B, respectively, they are converted into digital image signals by analog-to-digital (A/D) converters 122R, 122G and 122B, respectively. The A/D converters 122R, 122G and 122B each divide the dynamic range (a difference in the reading output between a pure white region and a pure black region of the document) of the image sensor 106 according to a bit number, thereby assigning levels of gradation according to the brightness of the document image.
For example, 8-bit-resolution A/D converters are capable of distinguishing a white to black gradation into 256 levels, while 10-bit-resolution A/D converters can differentiate the same gradation into 1024 levels. Thus, an image reading apparatus using A/D converters with RGB colors each having 8 bits can identify 24 bits, i.e., approximately 16.7 million colors, while an image reading apparatus using A/D converters with RGB colors each having 10 bits can distinguish 30 bits, i.e., about 1074 million colors.
The photodetectors 106R, 106G and 106B of the respective colors of the image sensor 106 are spaced apart from each other as noted above. Accordingly, in order to perform phase matching of the respective image signals before the signals are input into an image processing circuit 124, a (m+n)-line buffer memory 123R and an n-line buffer memory 123G are respectively provided at the rear stage of the A/D converters 122R and 122G, and the image signals R and G can be output simultaneously with the last-read B signal. In the image processing circuit 124, the image signals are subjected to processing, such as binary processing, for color correction. The resulting image signals are output to a machine 300, such as a personal computer, via an interface circuit 125.
There are several types of output states of the image signals from the image reading apparatus, and a suitable type can be selected according to the use of the read image. For example, when text is read by an optical character reader (OCR), or when a monochrome diagram is read, a monochrome binary image is appropriate. More specifically, a G signal, for example, among the RGB image signals is used and binarized with a threshold in the image processing circuit 124, and the binarized image data is selected. Further, when a photographic image is read and output to a monochrome printer, image data binarized using the G signal according to half tone processing, such as the dither method or the error diffusion method, is used. Multilevel (e.g. 24 bits, etc.) image data is suitably used for processing color images.
In most cases, the resolution of the A/D converters used in the image reading apparatus is comparable to the image processing performance of a computer to be connected to the apparatus (A/D converters with RGB colors each having 8 bits are employed in an image reading apparatus for use in a computer which is capable of processing 24-bit images). For achieving higher-precision gradation steps, however, some image reading apparatuses use A/D converters with a resolution higher than the image processing performance of the corresponding computer. In this type of image reading apparatus using, for example, A/D converters with RGB colors each having 10 bits, the respective RGB signals each having 10-bit gradation levels are converted into 8-bit signals in the above-described image processing circuit 124, and the 8-bit signals are output.
In the above-described 2:1 optical system, there are two factors which determine the precision of the reading magnification in the sub-scanning direction. One factor is moving precision of the lamp unit 103 and the mirror unit 104 (see FIG. 6A). The other factor is mounting precision of the optical system 102, for example, precision of the angle between the second and third mirrors M.sub.2 and M.sub.3 ; if this angle deviates from 90 degrees, a deviation of the reading position relative to the first mirror M.sub.1 is generated, as shown in FIG. 9, from the upper to lower edges in the sub-scanning direction. This further displaces the positional relationship of the reading position to the transparent-document reading light source L.sub.2. If there is any change in the light quantity at the reading position due to the above deviations, a document cannot be correctly illuminated, thereby failing to accurately reproduce the brightness of the document.
A large amount of light is required for precisely reading image information on a transparent document, in particular, a negative film. More specifically, it is necessary to accurately detect a density change in the information concerning a bright portion of a subject which is recorded in a dark portion of a negative film. To meet this requirement, a bright light source and a translucent plate having high transmittance are needed.
As noted above, there is generated a deviation in the positional relationship between the scanning optical system 102 (the lamp unit 103) and the transparent-document-reading light source L.sub.2 as the sub-scanning operation proceeds (see FIG. 6A). To avoid a change in the light quantity due to this deviation, a region in which the light quantity is uniform (hereinafter referred to as "a uniform-light-quantity region") on the document surface should be as large as possible, and accordingly, the diffusion coefficient of the translucent plate 201 should be as high as possible.
However, the transmittance and the diffusion coefficient of a translucent plate have a trade-off relationship. Increased transmittance decreases the uniform-light-quantity region. Thus, an appropriate trade-off between the two factors should be determined. There are also other factors which decrease the uniform-light-quantity region, such as skewing of the light source L.sub.2 itself, and a difference in tilting in the main scanning direction between the optical system 102 of the apparatus unit 100 and the transparent-document reading light source L.sub.2 (see FIG. 6A).